1. Field of the Invention
The present invention relates to an image pickup apparatus equipped with a so-called superimposing display function that display in-finder information such as marks and characters as an image superimposed on an object field image observed through an optical finder.
2. Description of the Related Art
Image pickup apparatuses including such a superimposing display function include ones provided with a liquid crystal display element disposed near a focusing screen on which an optical image (object field image) is formed by light from an image taking optical system, as an in-finder display element for displaying the in-finder information to be superimposed on the object field image. The in-finder display element includes a display pattern corresponding to the in-finder information and being formed by a transparent electrode. Switching between application and non-application of voltage to the display pattern enables switching between non-transmission (diffusion) and transmission of a liquid crystal area facing the display pattern, that is, switching between display and non-display of the in-finder information (ON and OFF of display).
Moreover, image pickup apparatuses including the optical finder include ones each of which measures information on luminance, color and the like of the object field image formed on the focusing screen by using light from the focusing screen (the information is hereinafter referred to as “object field information) and performs exposure calculation by using the object field information. The in-finder display element provided in such image pickup apparatuses may impede the measurement of the object field information truly necessary because the measurement is performed not only on the object field image but also on the superimposed in-finder information.
Japanese Patent Laid-Open No. 04-308825 discloses an image pickup apparatus that erases (turns off) display of the in-finder information on the in-finder display element in response to start of object field information measurement such that the displayed in-finder information may not impede the object field information measurement.
However, response speed of liquid crystal used in general liquid crystal display elements becomes slower as environmental temperature becomes lower. For example, although a so-called rise time for which the liquid crystal changes from a non-transmissive (diffusive) state of diffusing light to a transmissive state of transmitting the light is tens of milliseconds at a normal temperature of 25° C., it becomes significantly slow (long) to hundreds of milliseconds at a low temperature of −10° C. The same applies to a so-called descent time (or fall time) for which the liquid crystal changes from the transmissive state to the non-transmissive state.
Therefore, image pickup apparatuses, such as the one disclosed in Japanese Patent Laid-Open No. 04-308825, using a normal liquid crystal display element as the in-finder display element involve the following problem. In a case of performing control that once turns off display of the liquid crystal display element during a period of time (for example, 10 milliseconds) for which the object field information is measured and then turns it on, the measurement is little influenced by the above-mentioned change of the response speed of the liquid crystal under the normal temperature at which the response speed is fast. However, under the low temperature such as −10° C., since the liquid crystal does not sufficiently change from the non-transmissive state to the transmissive state (in other words, approximately maintains the non-transmissive state) within 10 microseconds for which the object field information is measured, the object field information cannot be accurately measured.